My different feedback tones:
Stand really close to your amp- screech
Boss Metalcore on my dirty channel (really a clean channel with gain @max)- slight feedback
Dunlop Jimi Hendrix Silicon Fuzzface into Boss Metalcore- screech with slight fuzz
Fuzzface into Metalcore on dirty channel- loud screech with slight fuzz and tons of static
Fuzzface into Metalcore on dirty channel with wah- pure audio chaos
These are the only 4 effects I use (dirty channel, wah, metalcore, fuzzface). My rig pretty much revolves around the ability to generate feedback. I love the little feedback I love it good.